The efficient functioning of the hip joint is extremely important to the well-being and mobility of the human body. Each hip joint is comprised by the upper portion of the femur which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within the acetabulum in the pelvis. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the acetabulum so that the ball of the femur and the hip bone rub together causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming misshapen.
Operations to replace the hip joint with an artificial implant are well-known and widely practiced. Generally, the hip prosthesis will be formed of two components, namely: an acetabular component which lines the acetabulum; and a femoral component which replaces the femoral head. The femoral component may be total femoral head replacement in which case the component includes a head, neck and a stem which in use in inserted into the end of a prepared femur. Alternatively, where appropriate, the femoral head component may be a resurfacing prosthesis which is attached to the head of the femur once it has been suitably machined.
In an operation to insert a prosthetic acetabulum in a patient's pelvis the surgeon first uses a reamer to cut a cavity of appropriate size in the patient's pelvis. An acetabular cup is then inserted into the cavity. By “appropriate size” is meant a size which is selected by the surgeon as being the most appropriate for that particular patient. Normally, it is desirable to retain as much of the original healthy bone surface as possible.
Commercially available acetabular cups are sold in a range of sizes to suit the needs of individual patients. Generally, acetabular cups are available in sizes of from 42 mm to 62 mm diameter with 2 mm increments between neighboring sizes.
There are a number of different types of prosthetic acetabular cups. One type of cup is those made from polyethylene. They are generally cemented into the acetabulum and require only light pressure to seat them in the cement.
One alternative cup type has a polyethylene liner unit for articulation with the femur and a metal shell for insertion into the pelvic cavity. These cups with metal shells may be implanted without cement such that they rely on a jam fit between the metal shell and the patient's acetabulum. However, in some arrangements, screws may be used to secure the cup shell in position in the pelvis before the liner is applied into position. The insertion of the metal shell into the pelvis requires considerable force. As the surgeon applies this force, there is a risk that the metal shell can become damaged or deformed. There is also a possibility that during the application of the force, the shell may be moved so that it is not in the optimum alignment in the acetabulum. Often the metal shells have outer surfaces or coatings which encourage bone to grow into them over time.
With this type of prosthesis, the polyethylene liner unit is snapped or screwed into the metal shell after the metal shell has been seated in the acetabulum. Thus the inner surface of the liner forms the socket part of the joint.
More recently, ceramics have been used to as an alternative to the plastics liner. In this arrangement, the metal shell, which is generally formed from titanium and which is of a similar thickness to the arrangement in which a polyethylene liner is used, is inserted into the acetabulum. The ceramic liner is then inserted into the shell. It can be difficult for the liner to be accurately aligned in the shell. In addition, this insertion of the liner does require the application of a considerable force which is usually applied by the surgeon using a mallet often via an insertion tool. Considerable force is generally required to achieve a successful interface. However, this force can damage the ceramic liner.
In order to get an optimum fit, it is necessary that the forces applied for both the insertion of the metal shell and for the ceramic liner are appropriate but not excessive. One problem however, is that to date there has been no understanding as to what forces are appropriate nor is there a means to ensure that the correct force is applied.
The surgeon is not generally able to apply a controlled amount of force applied. Some surgeons may not apply sufficient force in one hit and it may be necessary for a plurality of hits to be used. These may not all strike at the same angle and may not each apply the same force. Other surgeons may apply a much greater single strike. The force applied by the surgeon on, for example, an insertion tool may vary considerably and can be of the order of about 3 to 5 kN but can also be much higher and may even be of the order of about 35 kN.
Whilst very large forces may only be applied for small moments in time, of the order of seconds or fractions of a second, forces of this magnitude, or a plurality of forces of smaller magnitude may cause the shell to be deformed as it is inserted into the acetabulum. This is a particular risk in those arrangement s where the thickness of the shell is only from about 1 mm to about 3 mm thick. If the shell is deformed, it can become difficult or even impossible to insert the liner.
Additionally or alternatively, the liner may be incorrectly seated in the shell which can lead to various disadvantages. Not only is there a risk that where a portion of the liner stands above the rim of the cup, a point of irritation can be produced but also, there is a risk that material, such as wear debris, may congregate against the raised portion of the liner or against the wall of the cup in the area where the liner sits below the rim. This accumulation of debris may provide a site for post-operative infection. Even if the liner is correctly located and the shell is not deformed during the assembly process, it may become deformed on insertion of the prosthesis into the pelvis such that the shell may become spaced from the liner over at least a portion of the prosthesis.
Even if the surgeon is able to accurately seat the liner in the cup, there is a risk that during assembly debris may be caught between the liner and the cup which may effect the wear properties of the prosthesis. A further problem associated with the presence of debris, which may include fluids such as blood or fat, between the shell and liner is that in use, in vivo the presence of the debris may cause the shell and liner to move apart.
Without wishing to be bound by any particular theory, it will be understood where the shell and ceramic liner are held together by friction, debris, in particular fatty substances or blood, can interfere with the frictional interface between the outer surface of the liner and the inner surface of the shell such that there is a propensity for the liner to move out of the shell.
A further problem which may be encountered is that while inserting the liner in the shell it may become damaged. If this damage is a chip or crack on the outer surface of the liner, i.e. on the surface adjacent to the surface of the shell, its presence may not be noticed by the surgeon during assembly. However, its existence will be a point of weakness which can result in the prosthesis failing in use.
It is therefore desirable to provide an acetabular component which reduces the risk of liner misplacement and which has enhanced life expectancy arising, in part, through improved resistance to damage caused during impaction into the acetabulum. It is also desirable to provide an acetabular cup prosthesis which can be easily handled and inserted during surgery without damage to the acetabular cup prosthesis and which minimizes the risk of debris being trapped between the cup and the liner.